Recently, rapid developments of the electronics industry, various information communications including mobile communications, and mobile information technology (IT) products has led to an increase in the use of secondary batteries. Owing to techniques for producing hybrid automobiles, the demand for batteries having larger capacities and higher energy densities has started to increase. As a result, lithium (Li) secondary batteries having the highest performance, among batteries having large capacities and high energy densities, have been required more and more. To meet these requirements, it has become more necessary to improve performance of active materials among electrode materials.
Three essential components of lithium secondary batteries are a positive electrode, a negative electrode, and an electrolyte. Lithium transition metal compounds, such as LiCoO2, LiMn2O4, LiNiO2, Li(Mn,Ni,Co)O2, LiMnO2, or LiFePO4, may be mainly used as positive electrode active materials of the lithium secondary batteries. Lithium ions of these materials are intercalated into and deintercalated from crystalline structures to cause electrochemical reactions.
As application fields of lithium secondary batteries start from small-sized electronic products and cover a wide range, the efficiency of industrial activity of portable and mobile electronic products and hybrid automobiles has greatly improved. However, due to some disadvantages of batteries, such as thermal instability, high prices, and long manufacturing times, a large amount of research has been conducted into developing cheap, safe materials, reducing manufacturing process times, and improving economical efficiency.
The commonest material of positive electrode materials capable of causing improvements in price, safety, and capacity and producing the maximum effects, among battery materials, is LiCoO2. Although LiCoO2 has good conductivity and high performance, since LiCoO2 is expensive and problematic in safety, materials to replace cobalt (Co)-containing positive electrode materials are being studied. Among powerful candidates, LiFePO4, which has a theoretical capacity of about 170 mAh/g, provides theoretical capacities according to conditions and is superior to LiCoO2 in terms of price and safety.
However, the biggest disadvantage of LiFePO4 is low conductivity. LiFePO4 itself has low electrical conductivity, and large particles of LiFePO4 have low ionic conductivity. To solve this problem, research has been conducted on various methods, for example: a method of improving a diffusion speed of lithium ions by synthesizing uniformly distributed small-sized particles to enhance rate capability, or a method of coating LiFePO4 with carbon (C) to improve electrical conductivity.
However, to synthesize LiFePO4 having high crystallinity using a conventional method, such as a solid-state reaction, a sol-gel reaction, thermal synthesis, or a co-precipitation reaction, an annealing process should be performed at a high temperature. Also, a polyol method involves a cleaning process, a filtering process, and a drying process. Accordingly, a synthesis process may be complicated, incur high costs, and degrade time efficiency.